1. Field of the Invention
The present invention relates generally to the field of loudspeakers, and in particular, to a voice coil adaptor ring for mating the cone and spider for increased strength and efficiency.
2. Description of the Prior Art
With reference to FIG. 1, a conventional loudspeaker 20 generally comprises a support frame 22, a cone 24, a dust cap 26 bridging across the cone, a suspension system, a voice coil 40, a voice coil bobbin/former 43, and a vented pole piece 41. The voice coil 40 is wound about the voice coil former 43 such that an annular magnetic gap is defined between a top plate 37 and the magnet and the voice coil 40. The magnetic circuit linearly cycles or displaces the voice coil former 43 in this gap. In the conventional speaker, the cone shaped diaphragm (cone 24) is attached to the voice coil former above the coil 40 at its lower end and to the frame at its upper end. A suspension system comprising two elements connects to the frame and upper end of the cone, and to the frame and voice coil former, respectively.
The suspension system of the loudspeaker normally comprises two elements, the surround 28 (upper or outer suspension) and the spider 38 (lower or inner suspension). The surround 28 is a mechanical device which holds the outer edge of the diaphragm/cone of the loudspeaker and is often referred to as a "roll." Typically, the surround comprises a single, large, semi-circular corrugation constructed from either rubber, compressed foam rubber, or some similarly treated fabric. Surrounds may also be constructed from several other materials including corrugated cloth, paper, plastic, etc. One purpose of the surround is to help keep the cone 24 centered and to provide a portion of the restoring force that keeps voice coil in the gap defined between the pole piece and the top plate of the loudspeaker. The surround also provides a damped termination for the edge of the cone. A choice of thickness and material type for surround construction can greatly alter the response of the loudspeaker.
A spider 38 is commonly constructed from treated corrugated fabric. The spider 38 comprises a lower/inner suspension member that helps to keep the voice coil concentric to the pole piece. A portion of the restoring force that maintains the voice coil within the gap is also provided by the spider. Thus, the stiffness of the spider can greatly affect the loudspeaker's resonance. The spider also provides a barrier for keeping foreign particles away from the gap area.
In addition to controlling the linear motion of the cone, the surround, like the spider, acts as a major centering force for the loudspeaker's voice coil. The voice coil generally comprises a winding concentrically supported by a cylindrical voice coil former. The centering force provided by the roll and spider prevents the voice coil and former from rocking and rubbing against the pole piece or top plate. Rocking is undesirable because it can cause audible noise and/or damage to the driver.
Often a loudspeaker design can be best optimized by utilizing a voice coil with a smaller diameter. However, the smaller voice coil setup creates certain problems, especially when designing loudspeakers for low frequency reproduction. Thus, for larger diameter loudspeakers (typically 10 inches and above), small voice coil systems are not common. Accordingly, there are few, if any, existing cones tooled for the smaller diameter coil former. To incorporate a small voice coil system, the cone must be customized, adapted or re-tooled.
One disadvantage of mating a cone directly to a smaller voice coil is that a relatively small adhesive joint is made. Since the voice coil's diameter is much smaller, the gluing circumference is drastically reduced. Therefore, the designer must be concerned with the possibility of mechanical failures since the stress distribution around the glue joint is high. Because the spider attaches at this critical junction as well, spider joint stress also increases, introducing yet another possible failure mode.
Another problem associated with smaller voice coils occurs in the use of pole vents. Pole vents comprise holes bored directly through the pole piece within the motor structure. These vents are used to relieve air pressure that builds up beneath the dust cap. Without a pole vent, audible noise can be introduced as the trapped air tries to escape during large cone excursions. However, when using a small diameter voice coil, the amount of metal in the pole piece is very limited. This amount of steel can only support limited amount of magnetic flux. Consequently, using a pole piece with large amounts of metal removed for pole vents can radically alter the performance of the magnetic circuit.
A vented pole piece further affects the thermal behavior of the speaker. The steel contained in the pole piece provides an effective thermal sink for the voice coil. Machining a pole vent in the pole piece increases thermal resistance of the sink, lowering the power handling capability of the loudspeaker.
The mechanical integrity of the spider is also compromised when using a small voice coil. Spiders are typically made from resin treated cloth materials. When the inner diameter of the spider gets smaller, fewer strands of material intersect the cutout. Since the glue joint lies on this small circumference, very little spider material is captured. This places the spider material under greater stress than normal. This high-stress condition could cause the spider itself to fatigue prematurely. Since the spider is typically called on to center the moving assembly and limit cone motion at the extremes of excursion, a compromised spider could cause a catastrophic failure.
Rocking resistance is also compromised when using a smaller inner diameter voice coil. Rocking in a loudspeaker describes the moving assembly rotating in the vertical plane about a point located along its axis of motion. As a spider's inner diameter gets larger, the material along the inner diameter is required to deflect more when the moving assembly rotates a given amount (as during rocking). Consequently, a spider with a larger inner diameter will be more resistant to rocking because more energy is required to invoke a given angular change. It follows that using a small voice coil, and hence a small inner diameter spider, makes a given loudspeaker more susceptible to rocking related problems.
The smaller voice coil system further affects the cone's structural integrity. As a voice coil gets smaller, the cone angle increases (using a vertical axis as a reference), causing the cone to become flatter. As the cone begins to flatten, its mechanical strength drops. Increasing the cone angle increases the likelihood of audible degradation due to cone flexure. Normally, the only option available for preventing cone flexure is to increase the cone thickness and/or increase the cone depth. This decreases the cone angle and makes the cone wall more vertical. These solutions, however, are not desirable since increasing the cone depth requires a larger frame depth and using a thicker cone adds weight to the moving structure. Moreover, thicker cones and deeper frames require special tooling and make the speaker's mounting depth unattractive for certain applications.
Several loudspeaker designs are contemplated in the background art for improving speaker performance, stabilizing the speaker cone/diaphragm, and/or simplifying the manufacturing process. However, none of these references solve the above-noted problems. For example, Mitobe (U.S. Pat. No. 5,111,510) discloses a speaker and manufacturing method therefor including a diaphragm integrally combined with a first frame piece and a driver unit integrally combined with a second frame piece. Saiki et al. (U.S. Pat. No. 5,371,805) discloses a speaker and speaker system employing the same, comprising a diaphragm secured to a first periphery of an edge member and a frame secured to a second periphery of the edge member. Scholz (U.S. Pat. No. 5,323,469) discloses a conical loudspeaker having a conical stabilizing element joined between an underside of a speaker membrane and an outside surface of a speaker moving coil carrier. Kreitmeier (U.S. Pat. No. 5,424,496) discloses an electromagnetic converter comprising an internal magnet system, a moving coil and tubular segment. Kreitmeier (U.S. Pat. No. 4,764,968) discloses a disk-like diaphragm made from a conical plastic film and provided with vacuum formed support members which extend up to the disk-like radiating layer. Finally, Kobayashi (U.S. Pat. No. 4,118,605) discloses a coil mount structure comprising a cylindrical member, around one end portion of which a diaphragm edge is fixed, an inner peripheral edge portion where a damper is removably fixed, and an opposite end portion around which a coil is provided. Kobayashi, however, does not provide any structure for ventilating air pressure from beneath the dust cap or a structure for creating a secure joint between the diaphragm/cone, spider, and/or voice coil. The present invention, by way of contrast, is directed to an adaptor ring, the structure of which facilitates a stronger adhesive joint between the cone, spider, and voice coil bobbin or former, and a means for venting air pressure buildup.
The above-noted background art neither solves or addresses the problems contemplated by the present invention. Accordingly, there remains a need for a loudspeaker capable of providing improved structural joints between the speaker cone, spider, and voice coil former, allowing the use of smaller voice coil systems and providing ventilation in the speaker without forfeiting performance. The instant invention addresses the needs in the art by providing a voice coil adaptor ring that provides increased stability to the speaker cone, spider, and voice coil former, and that facilitates the reliable use of smaller voice coils in loudspeaker designs, including low frequency speakers. The instant invention also addresses the need for improved ventilation.